Frequency control



1944. g. o. 'PEITEYRS'ON 2,354,827

FREQUENCY CONTROL Filed May 23, 1941 1:90 I a? I ee 34 INVENTOR 5 /YarpLZQP 0n Fra aw ATTORNEY Patented Aug. 1, 1944 2,354,827 FREQUENCY CONTROL Harold OlPetersou, Riverhead, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application May 23, 1941, Serial No. 394,761

' 9 Claims.

This application concerns automatic frequency control, methods and means for mark and space spaced wave keying receiver systems, and makes use of dual discriminators.

In mark and space wave signalling the transmitter frequency is in general at either one or the other of two frequencies for various lengths of time. This method of signalling is also referred to as wave length modulation wherein Wave energy is keyed or shifted from one frequency (which maybe called thespacing frequency) to another frequency (which may be called the marking frequency) and is particularly applicable to telegraphy transmission including time division multiplex teletype and photoradio using C. F. V. D.

An object of my invention is to provide a system which will give automatic frequency control during either one of the said two conditions, i. 8., continuously during marking and spacing. To accomplish this object I provide means which produces a control potential characteristic of undesired frequency deviations at all times irrespective of the fact that the current characteristic of received wave alternates between two frequencies and the amplitude of one of said currents goes to zero or an unusable value at the time the amplitude of the other of said currents becomes of usable value and vice versa.

In describing my invention in detail reference will be made to the drawing wherein Figure 1 shows a schematic diagram of this system, Figures 2 and 3 show frequency characteristics of certain portions of the circuit of Figure 1.

The spaced wave signal transmission is received on antenna I, and is heterodyned in high frequency unit 2 against oscillatory wave energy from 3 to produce corresponding Wave energy of a first intermediate frequency which. is conducted to intermediate frequency circuits in unit 4 where it is again heterodyned against oscillatory energy from unit 5 to produce a second intermediate frequency which is passed therein through amplifier and selective stages an'dthence into limiter 6. The output of limiter 6 is passed through a frequency modulation detecting circuit in 28 the output of which is conducted to the utilization circuits or devices, which may include recording apparatus. Also, partof the output of the limiter in 6 is conducted to filter circuits 1 and 8, which have band-pass characteristics as shown in Figure 2. It will be noted that one of these filters passes the space wave frequency S, and the side frequencies grouped ther eabout, and the other filter passes the marking wave frequency M, and

the side frequencies grouped thereabout. The currents passed by 1 and 8 alternate between low or unusable and high or usable values.

The output of filter I goes to a frequency discriminator circuit consisting of selective circuits 9 and I0 feeding diodes l3 and I4. In a preferred embodiment the selective circuits 9 and ID each comprise an inductance and condenser in parallel, one circuit being tuned to a frequency slightly above the frequency of say the space wave, the other circuit being tuned to a frequency slightly below the frequency of the space wave. The output of filter 8 goes to a discriminator consisting of selective circuits H and I2 feeding diodes I5 and IS; The selective circuits H and [2 are in a preferred modification also comprised of parallel reactances ofi tuned with respect to say the mark wave. Rectifier I4 is in a series circuit including resistance I8 and tuned circuit l0. Rectifier I3 is in a circuit including resistance II. The polarity'of the rectifiers are opposed so that opposing potentials are produced in the series resistances l1 and I8. Rectifiers l5 and I6 and circuits H and I2 are connected in a similar manner to resistances I! and i8. Resistors l1 and [8 ar by-passed by condensers l9 and 20. The arrangement as connected is such that if either the marking or spacing wave is on control potentials are developed across resistances H and I8.

The differential direct current voltage generated across the load resistances l1 and I8 is transmitted to control unit 2| which operates upon the frequency of oscillator 3 so as to compensate properly for any drift of the wave energy reaching 6 from its desired normal frequency irrespective of whether such drift is due to changes in transmitter or receiver oscillator frequencies or other causes. lator in unit 2| may comprise any known means for controlling the frequency of the oscillations supplied from unit 3. Preferably the means in unit 2| includes amotor driving a tuning element such as, for example, a reactance in the circuit of the wave generating means in unit 3 with the motor controlled as to direction and extent of rotation by the potential developed across resistances l1 and I8. I also prefer to use an electron tube system for controlling the motor with the tube system controlledby thepotential developed across resistances I1 and I8, the arrangementbeing as described in Crosby U. S. application Serial No. 393,339, filed May 14, 1941. However, if desired the unit 2| may include a reactance tube modulator, coupled with an oscillator in unit 3,

and controlled by potentials supplied from re- The control or modu-J 136,578, filed April 13,

current characteristic of diode l and 24 represents the current versus frequency characteristic of diode I6. It will be noted that one pair of diodes takes care of automatic frequency control and the other pair of diodes take care of automatic frequency control during the interval when the transmitter is on the mark frequency M. The action of passing the outputs of these diodes through common load resistors l7 and i8 is to give a switching action such that the control will come from thediode having. the greatest voltage impressed.

A suitable time, constant circuit TC is included in the circuit controlling the oscillation generator in 3 so that it will not tend to follow at signalling frequency. .JIhis circuit may comprise a condenser C and resistance R included in theconnection between resistances 11;.and i8 and the means in unit 2|. I

Other forms of frequencydiscrimination cir of which changes in a manner which corresponds.

to-changes in frequency of energy comprising two waves of different frequency and their side bands resulting from modulation of said waves including modulation of one or the other thereof to substantially zero wave'amplitude at' all times, selective circuits for substantially separating said waves one from the other, a frequency discriminator circuit coupled to each of said selective circuits, an impedance, and a'separate' differential rectifier system coupling each of said discriminator circuits to said impedance whereby currents fiow in said impedance due to rectification of the stronger of saidwaves to produce in said impedance a potential which varies with changes in the frequency of said rectified wave, the arrangement being such that the potentials produced by current from the rectifier'systems are of likepolarity, said separate differential rectifier systems and frequency discriminator cir-- cuits each including means for producing voltages which differ in direction as'the frequency of'the wave applied to the discriminator circuits vary in frequency'above or below a predetermined mean signaling which includes these steps, limiting the amplitude of said wave energies to remove therefrom amplitude variations, selecting limited waveenergy of a frequency of the orderof said'fi-rst frequency, selecting limited wave energy of a frequency of the order of said second frequency, producing by means of either of said selected limited wave energies a potential the polarity of which changes when the frequency of said selected wave energy producing said potential changes relative to a selected frequency, and controlling the frequencies of said energies in accordance with said produced potentials.

3. In means for automatically tuning a tunable receiver of wave energy, comprising a marking frequency and a spacing frequency, the frequencies of which may depart from their normal frequencies, a filter for passing wave energy of said markingfre uency, a second filter for passing when the transmitter is on the space frequency S q wave energyof said spacing frequency, a frequency discriminating circuit excited by wave energy passed by said first filter, a frequency in accordance with the potential drop in said impedance.

4. In means for automatically tuning a tunable receiver of wave energy, comprising a marking frequency and a spacing frequency, the frequencies of which may depart from selected frequencies, a wave amplitude limiter excited by said wave energy, a first filter tuned to pass wave energy of said marking frequency coupled to said limiter, a second filter tuned to pass wave energy of said spacing frequency coupled to said limiter, a first discriminator circuit coupled to one of said filters, a second discriminator coupled to the other of said filters, a load impedance, a pair of differentially arranged rectifiers coupled to each of said discriminator circuits and having as an output impedance said load impedance, each discriminator and the pair of diiferentially arranged rectifiers coupled thereto including means for producing voltages which differ in direction as the frequency of the wave energy applied to the discriminator varies in frequency from a predetermined mean frequency the connections being such that the rectified currents of said pairs of difierential rectifiers flow in said load impedance to produce therein a potential which varies in accordance with changes in the frequency of said wave energy, and means for controlling the tuning of said receiver in accordance with said potential.

5. In means for continuously producing potentials characteristic of the frequency of wave energ comprising alternatively produced waves of a first and second frequency, an impedance, a first frequency discriminator circuit responsive to wave energy of said first frequency, a second frequency discriminatorcircuit responsive to said.

wave energy of said second frequency, a first differential rectifier system coupled to said first.

discriminator circuit and including as a load said impedance, and a second diiferential rectifier system' coupled to said second discriminator circuit and including as a load said impedance, the polarity of saidrectifier systems with respect to said impedance being the same, each of said differential rectifier systems and the discriminator circuit coupled thereto including means for producing voltages which differ in direction as the frequency of the wave energy applied to the discriminator circuit varies in frequency from a predetermined mean frequency.

6. In means fOr automatically tuning a receiver having a tunable circuit and being adapted to receive wave energy, comprising alternatively transmitted marking frequency and spacing frequency waves separated by a substantial space in the frequency spectrum, means for receiving said Wave energy, said means including means for tuning the wave energy receiving means, a wave amplitude limiter coupled to said means, a path which passes freely said marking frequency wave and side frequencies grouped thereabout, a second path which passes freely said spacing frequency wave and side frequencies grouped therealoout, connections coupling the input of each of said paths to said amplitude limiter an impedance, two differentially connected rectifier systems each including said impedance, a frequency discriminating circuit coupling one of said differential rectifier systems to one of said paths, a second frequency discriminating circuit coupling the other of said differential rectifier systems to the other of said paths, the arrangement being such that rectified currents from either of the differential rectifier systems produce potentials of like polarity in said impedance, each of said discriminator circuits and the differential rectifier system coupled thereto including means for producing a potential the polarity of which reverses as the frequency of the Wave energy applied to the discriminator circuit varies above or below a selected frequency and means actuated by potentials produced across said impedance for controlling the tuning of said tunable circuit.

7. In means for automatically tuning a receiver having a tunable circuit and being adapted to receive spaced wave energy comprising alternatively transmitted marking frequency waves and spacing frequency waves separated by a substantial space in the frequency spectrum, means for receiving and amplifying said wave energy, said means including a variable reactance for tuning the wave energy receiving means, a filter which passes freely said marking frequency wave and side frequencies grouped thereabout, a. second filter which passes freely said spacing frequency wave and side frequencies grouped thereabout, means coupling the input of each of said filters to said first-named means, a pair of rectifiers for each filter, a frequency discriminating circuit coupling one pair of said rectifiers to one of said filters, a second frequency discriminating circuit coupling the other pair of said rectifiers to the other of said filter circuits, a pair of resistances connected in series, a coupling between each of said resistances and a rectifier of each of said pairs of rectifiers, the arrangement being such that the rectifiers of each pair are connected in opposed relation in series with said series resistances, the polarity of the pairs of rectifiers as connected to the series resistances being similar and means actuated by potentials produced across said series resistances for controlling said variable reactance to thereby control the tuning of said receiver.

8. In a tuning system for a receiver, for signals comprising wave energy keyed in accordance with signals from a first frequency to a second frequency, having a tuning element, a first filter for passing current characteristic of wave energy of said first frequency, a second filter for passing current characteristic of wave energy of said second frequency, a first frequency discriminator coupled to said first filter, a second frequency discriminator coupled to said second filter, two impedances in series, a first pair of rectifiers, connections coupling said pair of rectifiers to said first discriminator and connecting the rectifiers of the first pair differentially to said series impedances, a second pair of rectifiers, connections coupling said second pair of rectifiers to said second discriminator and connecting the rectifiers of the second pair differentially to said series impedances, the arrangement being such that similar changes in frequency of either of said currents produces similar changes in the resultant of the rectified current through said impedances, and connections for controlling said tuning element in accordance with variations in the resultant poential drop in said impedances.

9. In a system for producing potentials characteristic of the departure of wave energy keyed in accordance with signals from a first frequency to a second frequency, a first filter for passing current characteristic of the wave energy of said first frequency, a second filter for passing current characteristic of the wave energy of said second frequency, said first and second filters each having an input on which said characteristic currents are impressed, said filters each having an output, a first frequency discriminator circuit having an input coupled to said first filter, a second frequency discriminator having an input coupled to said second filter, said first and second discriminator circuits each having an output, two impedances in series, a first pair of rectifiers, connections coupling said first pair of rectifiers to the output of said first discriminator and connecting the rectifiers of the first pair differentially to said series impedances, a second pair of rectifiers, connections coupling said second pair of rectifiers to the output of said second discriminator circuit and connecting the rectifiers of the second pair difierentially to said series impedances, the arrangement being such that the rectified currents produced by the rectifiers of each pair of rectifiers in the two impedances are opposed in polarit while the resultants of the rectified currents supplied by the respective pairs of rectifiers in the two impedances are of like polarity.

HAROLD O. PETERSON. 

